Using WordPress 4.7.2, ACF 4.4.11 and ACF: Location Field 1.0.0, the acf/fields/google_map/api filter did not work for me. What did work was to directly edit acf-location-field-master/location-v3.php and acf-location-field-master/location-v4.php to alter function admin_head() and function input_admin_head() to specify the API key:
echo '<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?v=3.exp&key=[your key]"></script>';
To fix front end, I added same script link to header.php.
